As long as you can look into it from the street you'll have some benefit at least. Walking along other people's gardens can be quite relaxing too as long as high walls are not allowed.
There’s a mix - some are walled off, some internal to housing (interior of a block), some just fenced off.
The Eaton Square viewed on Google Maps shows a variety. The main square is shoulder-high iron fencing. But many blocks have walked or internal gardens.
It’s nice that they exist. But it’s not so nice that they’re a perk for the rich and not accessible by an average resident.
And AFAIK, there is no planning rule that requires they be added with new (re-)development. Only maintaining what’s there.